More and more attention is paid to detection of explosives, drugs or the like in objects such as luggage. Some detection measures, for example, Computer Tomography (CT) detection technology, may obtain important information such as a spatial position distribution, density, mass, an effective atomic number or the like of various materials in the luggage, so as to recognize categories of different materials. When a suspicious material is detected by a system, an alarm is generated and the suspicious material is delivered to a detection apparatus in next stage for detection or the suspicious material is detected manually.
However, it is still of a high error rate to judge whether a certain material is an explosive by using information such as density, an atomic number or the like. In order to reduce the error rate of the whole system, reduce a number of manual detection operations and improve reliability of the system, a detection system based on coherent X-ray scattering is connected in series a CT detection system, which may significantly reduce the error rate of the system.
The coherent X-ray scattering (X-ray diffraction) technology is used to detect materials (which mostly are crystal materials), and is primarily based on the Bragg diffraction equation as follows:
                              n          ⁢                                          ⁢          λ                =                              2            ⁢                                                  ⁢            d            ⁢                                                  ⁢            sin            ⁢                                                  ⁢                          (                              θ                /                2                            )                                =                      nhc            E                                              (        1        )            
wherein n is a diffraction emphasis level, and generally satisfies n=1 in explosive detection; λ is a wavelength of an incident ray; d is a lattice spacing, and is also a lattice constant; θ is a deflection angle after rays are scattered; h is a Planck constant; c is a velocity of light; and E is energy of incident photons. When various parameters satisfy the above equation, coherence emphasis occurs, the corresponding scattering is elastic scattering, and the energy of the X photons is unchanged.
In a diffraction pattern based on an energy distribution, an angle θ at which the measurement is implemented by the detector is fixed, i.e., the energy spectrum of the scattered X-rays is measured at a fixed scattering angle. The lattice constant d and the energy E of the incident photons which satisfy the above equation are in a one-to-one relationship. Thus, fingerprint features of the crystal materials, i.e., lattice constants d1, d2, . . . dn may be determined according to the peak positions of the energy spectrums E1, E2, . . . En, so that different materials may be recognized. For example, typical explosives primarily include different crystal materials, and the crystal types are recognized according to the lattice constants. Therefore, this method is an effective explosive detection measure.
A single-energy X-ray source may also be used to count X photons at different scattering angles. The crystal information is obtained according to a one-to-one relationship between θ and d. This method may reduce the requirements for the detector, but has higher requirements for mono-chromaticity of the light source. In addition, it is inefficient to change an angle for measurement. This method is applied in an experiential device, but is infrequently used in practical designs and applications.
A detection method based on an inverted fan-shaped beam is proposed. A system using an inverted fan-shaped beam achieves measurement in a fixed manner by using a few detectors. In the inverted fan-shaped structure, scattered rays from objects in different positions in a detection plane which are perpendicular to a direction of a beam of rays are converged to a point on the detectors, which results in superposition of spectral lines of objects in two positions. In order to obtain information of various pixels in a section plane of materials, multiple light source points cannot illuminate at the same time, and need to emit rays in a certain order. This results in significant degradation of intensity of rays in the detection plane at any time and a relatively low signal-to-noise ratio of data measured by the system in a case that the materials pass through the detection plane at a certain speed.